Usage of Slangs in posts
Instead of using "u," we should type "you."
I agree that instant messaging abbreviations have no place in writing. However, there are instances where slang can be used in dialogue. In order for dialogue to appear natural, the writer should write as he would actually speak. I don't use chatspeak or messaging abbreviations in my writing. I don't even make a habit of using it in my replies to other people's work, but it is used so commonly that people sometimes forget this is not proper for writing.
'yeah' is not used in dialogue because it is so common in speech.
Slang cannot be a substitutes for normal or proper language, but most people can understand the context more often than not. Slang can also filter into normal everyday conversations. I think it's mostly used by teenagers to have a cute way of writing to each other and in that sense, it's fine. Let us limit our usage to conversations when you are sure that it is understood the way it is supposed to be and not to posts. This is just my personal opinion and evey one is free to say what they feel.